Flow control method and device

ABSTRACT

An arterio-venous graft  16  is provided with a constriction device  20  near its arterial end. The constriction device  20  is used to reduce the flow through the AV graft under normal conditions and to relieve the constriction when high flow through the AV graft is required, such as for vascular access for hemodialysis.

This invention relates to a method of flow control of a bodily vessel,for example for use in an arterio-venous graft, hereinafter referred toas an AV graft. The invention also relates to a device for controllingflow in a bodily vessel, such as an AV graft, and a combination of sucha device and a graft.

Patients with kidney disease, particularly those with end stage renaldisease (ESRD), require hemodialysis in order to remove metabolites andthe like from their blood stream. This can be a very time-consumingprocess, but the time can be lessened by providing a large blood flow tothe hemodialysis machine. Even though this is done, hemodialysis canstill take about four hours and is needed about three times a week.

In order to provide high blood flow to and from the hemodialysismachine, vascular access with high blood flow is needed. One method ofproviding this is illustrated in FIG. 1. An artery 10 and a vein 12 arelocated in the arm 14 of the patient. A vessel 16, known as an AV graftor shunt, is grafted to connect the artery 10 and vein 12. As the AVgraft 16 is a direct connection between the artery 10 and vein 12 andhas a relatively large cross-sectional area, a high flow through itoccurs. The direction of flow is indicated by the arrows in FIG. 1.Catheters (not shown) can be connected to the AV graft 16, whenhemodialysis is required. The catheters can tap into the high flowthrough the AV graft 16 to provide a high flow to and from thehemodialysis machine.

However, there are also considerable problems with this technique. Oneof these, illustrated in FIG. 2, is that stenosis occurs at the outflowtract where the AV graft 16 is connected to the vein 12, that is at thevenous anastomosis side of the graft. The stenosis 18 is an unnaturalnarrowing of the vessel, and if unopened by angioplasty, the stenosisprogresses until the vein is completely blocked. The stenosis is due toneo-intimal hyperplasia, that is the response of the vessel to theabnormal conditions. Various mechanisms are considered as possiblycontributing to the stenosis development. The flow through the vein istypically 10 to 20 times higher than normal. This leads to turbulenceand flow separation such that the flow is not smooth or laminar, and thestenosis develops as a result. Another factor is that the vein isexposed to a higher blood pressure than normal, because it is directlyconnected to the artery. The blood pressure in an artery is typically100 mm Hg, whereas the blood pressure in a vein is typically 5 mm Hg.The vein tends to arterialise in response to this, for example bythickening of the vein wall and this may contribute to the stenosis. Afurther possible factor is that, in the presence of the graft, the flowin the vein is pulsatile. There is a significant compliance mismatchbetween the AV graft, which, if synthetic, is quasi-rigid, and the veinwhich is compliant. The pulsatile flow produces an oscillating stressconcentration at the junction, i.e. suture line, between the AV graft 16and the vein 12. Although the suture usually does not fail, the stenosismay be in response to the oscillating stress concentrated at thejunction.

This is a considerable problem. In 90% of AV grafts, stenosis developsat the venous anastomosis side. AV graft survival is around only 1.5years. Conventionally, alleviation of this problem requires surgery,such as angioplasty to remove the stenosis or surgery to implant a newAV graft in a different limb of the patient.

A further problem is that the AV graft 16 effectively provides a shortcircuit between the artery 10 and vein 12 and the high flow through theAV graft 16 requires a huge additional cardiac output. Normal cardiacoutput is typically 5 litres per minute, but with the AV graft in placethis can increase to 7 litres per minute. This large additional cardiacoutput can be very problematic indeed, and can result in fatal cardiacfailure for about 5% of AV graft patients.

According to the present invention there is provided a method of flowcontrol in a AV graft or AV fistula used for vascular access for anextracorporeal circuit, said method comprising the steps of:

-   -   (a) applying partial constriction to a vessel to provide a        reduced flow through said AV graft or AV fistula, when flow        through said extracorporeal circuit is not occurring; and;    -   (b) changing the degree of constriction, to modify the flow        through the AV graft or AV fistula, when flow through said        extracorporeal circuit is to occur.

Applying partial constriction can reduce or eliminate turbulence, andlower the blood pressure in the vein. The constriction can also act as astrong wave reflector to reduce or eliminate the pulsatile flow at thevenous anastomosis. All of these can alleviate stenosis, prolong thelife of the AV graft or AV fistula and reduce the necessary cardiacoutput. Changing the degree of constriction when flow through saidextracorporeal circuit is to occur enables a high flow to be providedfor vascular access.

The constriction of the vessel is only partial, preferably to maintain areduced but significant residual flow through the AV graft to avoidthrombosis, and to keep the vein matured and able to handle the highflow when necessary.

Preferably the constriction is applied over an elongate portion of thevessel. This enables the flow control to be achieved by viscousdissipation in favour of turbulent dissipation.

Preferably the constriction is applied at a plurality of positions alongthe vessel and/or the profile of the constriction is controlled alongits length. This enables turbulence caused by the constriction to beminimised.

Preferably the constriction reduces the cross-sectional area of thelumen of the vessel, but maintains the length of the perimeter thereof,again to favour viscous dissipation.

Preferably, when applying the constriction to the vessel, the flow atthe venous anastomosis of the AV graft or AV fistula is monitored sothat when constricted, the flow is maintained at a level below the onsetof turbulence.

Preferably the vessel is an AV graft.

Preferably the constricting step comprises constricting said AV graft atits arterial end. This enables any turbulence caused by the constrictionto subside before the blood flow reaches the venous anastomosis.

The invention provides a device for controlling flow in an AV graft orAV fistula used for vascular access for an extracorporeal circuit, saiddevice comprising:

-   -   a) means for applying partial constriction to a vessel, to        provide a reduced flow through said AV graft or Av fistula, when        flow through said extracorporeal circuit is to occur; and    -   b) means for changing the degree of constriction, to modify the        flow through the AV graft or AV fistula, when flow through said        extracorporeal circuit is to occur.

The invention also provides a device, for controlling flow in a bodilyvessel, said device comprising an actuator for releasably constrictingsaid bodily vessel; and a rotatable member for driving said actuator.

Preferably the rotatable member comprises a drive shaft of a motor orcomprises a rotor rotatable by an externally applied magnetic field.

Preferably the motor is an electrical micromotor.

The invention also provides a device, for controlling flow in a bodilyvessel, said device comprising a deformable member which is reversiblydeformable by a change in temperature or magnetic field; and an actuatoracted on by said deformable member for releasably constricting saidbodily vessel, wherein said deformable member is deformable between afirst state in which said actuator applies constriction to said bodilyvessel, and a second state in which said actuator reduces saidconstriction of said bodily vessel.

Preferably the thermally deformable member comprises a shape-memorymaterial or a liquid filled capsule.

Preferably the device of the invention further comprises an antenna forreceiving signals for controlling the actuator. This avoids the need foraccess to the device through the skin and the potential risk ofinfection.

Preferably the device further comprises a converter for converting radiofrequency energy received by the antenna into energy for powering thedevice to operate the actuator. This has the advantage of avoiding theneed for an internal power source, such as a battery, in the device, andradio frequency activated devices are NMR-proof.

The invention further provides a device, for controlling flow in abodily vessel, said device comprising an actuator for releasablyconstricting said bodily vessel, wherein said actuator comprises a cliphaving two constriction portions with an adjustable separationtherebetween for accommodating said bodily vessel and a control portionfor releasably holding said two constriction portions such that saidseparation is held at least one predetermined amount.

Preferably the constriction portions are integrally formed as one memberwhich makes the device simple and cheap to fabricate.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a human lower arm, illustrating aconventional AV graft in situ;

FIG. 2 is a close-up view of the venous anastomosis of FIG. 1,illustrating a problem associated with the AV graft of FIG. 1;

FIG. 3 is a schematic view of a human lower arm, illustrating anarrangement according to the present invention;

FIGS. 4(a) and 4(b) are schematic cross-sectional views of a firstembodiment of apparatus according to the present invention, shownapplied to an AV graft;

FIG. 4(c) is a plan view of a deflectable membrane of an embodiment ofthe invention;

FIG. 5 shows a second embodiment of an apparatus according to thepresent invention;

FIGS. 6(a) and 6(b) show a third embodiment of an apparatus according tothe invention in cross-section and plan view, respectively;

FIGS. 7 and 8 show cross-sectional views of fourth and fifth embodimentsof apparatus according to the invention;

FIGS. 9, 10 and 11 are explanatory diagrams for illustrating furtheraspects of the present invention;

FIG. 12 illustrates schematically an embodiment of the inventionincorporating an electromagnetic flow measurement system; and

FIG. 13 illustrates an application of the invention to a Blalock-Taussigshunt.

FIG. 3 shows an arrangement according to the present invention, withcorresponding parts labelled the same as in FIG. 1. The AV graft 16 maybe an artificial vessel, for example, made of PTFE or GORE-TEX™ or othersynthetic material, or the AV graft may be an autologous graft. Asillustrated in FIG. 3, the AV graft 16 is connected to an artery 10 andvein 12 in the arm 14 of the patient. However, the AV graft 16 may, ofcourse, be located in other parts of the body, for example the leg,groin or neck.

A device 20 is provided for controlling blood flow in the AV graft 16.During the normal activities of the patient, the device 20 is used toconstrict the AV graft 16 such that there is a reduced or residual flowtherethrough. When flow through an extracorporeal circuit, such as ahemodialysis machine, is required, the degree of constriction isreduced, partially or fully, so that there is an increased, high flowthrough the AV graft 16. Catheters (not shown) can tap into the highflow in the AV graft 16 to provide high flow to and from a hemodialysismachine. The catheters may be upstream or downstream of the device 20 ormay be provided on opposite sides of the device 20. A single catheterwith a double lumen may also be used for flow to and from the AV graft16.

As illustrated in FIG. 3, the constriction device 20 is used toconstrict the AV graft 16 at its upstream end, in the vicinity of itsconnection with the artery 10. Preferably the constriction device 20 iswholly implanted within the patient and an external controller 22 isused telemetrically to control the constriction device 20.

When high flow through the AV graft 16 is no longer required, theconstriction device 20 is used to re-apply constriction to reduce bloodflow. A turbulence measuring device 24, 26 may be used to monitorturbulence in the vicinity of the venous anastomosis while the flowthrough the AV graft 16 is being reduced. As the degree of constrictionis increased, the flow rate reduces such that a level will be reached atwhich turbulent flow substantially ceases to be detected by theturbulence measuring device. When this occurs, further change inconstriction can be stopped and the flow maintained at that level belowthe onset of turbulence. Alternatively, the constriction may beincreased until the turbulence has been diminished to a predeterminedlevel, but not totally abolished. Preferably this diminished turbulenceintensity is below the level at which stenosis may occur, but the flowrate is still sufficient to keep the vein matured. In this way anoptimal quiescent flow can be established in the vicinity of the venousanastomosis side of the AV graft.

The turbulence measuring device 24, 26 can be a conventional Dopplerdevice or a phonoangiographer and may advantageously be connected to thecontroller 22 or constriction device 20 automatically to controladjustment of the flow rate, or this may be done manually.

Further features of the method of the present invention will be apparentfrom the following description of devices according to the invention.

FIGS. 4(a) and 4(b) are longitudinal and transverse cross-sections,respectively, of a constriction device 20 and control device 22. Thecontrol device 22 has an antenna 30 for transmitting signals to anantenna 32 provided on the constriction device 20. The antennae 30, 32are electromagnetically coupled to each other, but are of course onopposite sides of the skin (not shown) of the patient. A receiver 34connected to antenna 32 sends electrical power to a motor 36 in responseto the transmitted signal.

The constriction device 20 may contain an internal power source, such asa battery, which is controlled by the receiver 34 to deliver electricalpower to the motor 36. Alternatively, the receiver 34 may comprise aradio frequency to DC converter and modulator, in which case radiofrequency signals emitted by the antenna 30 are picked up by the antenna32 and these signals are converted by the receiver 34 into electricalpower to drive the motor 36, rather than the signals being used tocontrol an internal power source of the device, thereby eliminating theneed for batteries in the device which would need to be replacedperiodically.

The motor 36 is a miniature motor, also known as a micro-machine, andwhen provided with electrical power it can be used to rotate a driveshaft 38 in either direction, or in one direction only, provided thatthe actuator performs a periodic displacement even if the micromotorshaft 38 always turns in the same direction. The dimensions of themicromotor 36 are sufficiently small to enable it to be encapsulated inan implantable enclosure, for example the motor may be 2 mm thick and 15mm long. A rotary to linear transmission 40 converts the rotation of thedrive shaft 38 into linear motion of an actuator comprising members 42,44 and 46. Members 42 and 44 are rods or bars and member 46 is, forexample, a fine titanium membrane that is in contact with the AV graft16 or presses upon the AV graft through an intermediate material.

As shown in FIGS. 4(a) and 4(b), the actuator 42, 44, 46 is constrictingthe AV graft 16, such that the cross-sectional area of its lumen 48 isreduced. By sending appropriate signals, and through action of the motor36, the constriction can be relieved by motion of the actuator, whenhigh flow is required, and the position of the membrane 46 in this highflow state is indicated by the dashed line 50.

The constriction device 20 is encapsulated in an enclosure 52, such as atitanium or ceramic box, through which the AV graft can pass, or intowhich the AV graft can be slotted sideways. The antenna 32 asillustrated in FIGS. 4(a) and 4(b) is located outside the enclosure 52so that it is not screened by the enclosure and to enable the antenna tobe placed under the skin for optimal RF wave reception. This arrangementof having the antenna 32 external to and optionally remote from theenclosure 52 can be advantageous for cases in which the constrictiondevice 20 is implanted deep within the body and the RF waves from theexternal control unit have a maximum penetration depth of 2 to 4 cm.Alternatively, for situations in which the constriction device 20 can beimplanted just under the skin or not too deep, the antenna 32 can beinternal, i.e. encapsulated within the enclosure 52 of the constrictiondevice 20. In this alternative embodiment, the enclosure 52 or at leastpart of the enclosure 52 is non-metallic, for example ceramic or plasticto avoid screening of the RF waves. Having the antenna internal orintegral to the enclosure 52 of the constriction device 20 isadvantageous in simplifying the implantation of the device within thebody.

The device may optionally include a sensor, not shown, such as a sensorfor measuring the position of the actuator or for counting the number ofrevolutions of the drive shaft 38. Sensors for measuring flow,turbulence or pressure may also be included. Information from thesensor(s) can then be transmitted from the constriction device 20 to thecontrol device 22 via the antennae 30, 32, so that the controller 22 cancontrol the constriction more precisely.

FIG. 5 illustrates an alternative constriction device 20 in the form ofa clip. The actuator of the device comprises a pair of constrictionportions 60, 62 separated by a gap through which the AV graft 16 passes.The separation between the constriction portions 60, 62 can be reducedby applying pressure to the skin 64 of the patient to constrict the AVgraft 16. A control portion 66 comprises a series of grooves or notchesengageable by an insertion portion 68 of the constriction portion 60.Pressure applied to the skin 64 moves the insertion portion 68 from theposition shown in FIG. 5 into successively lower notches. When therequired level of constriction is achieved, the engagement of theinsertion portion 68 in the particular notch of the control portion 66maintains that level of constriction.

A pressing device 70 may be used for this process and may comprise asensor that detects the motion of the insertion portion 68 from onenotch to the next so that the position of the constriction portions isknown and an optimal level of constriction applied.

When high flow through the AV graft 16 is required, the constriction canbe reduced by again applying pressure to the skin of the patient, butthis time by pressing on a release portion 72. This splays the controlportion 66 so that the insertion 68 disengages from the notches and theopening between the constriction portions 60, 62 increases.

As shown in FIG. 5, the constriction device 20 is formed from a singlepiece, such as by moulding it from a biologically compatible plasticsmaterial. This makes it very simple and cheap to fabricate.

Another embodiment of the constriction device is shown in FIGS. 6(a) and6(b). It comprises an actuator plate 80, within an enclosure 82, forsqueezing on the AV graft 16. A rotor 84 is screwed onto a threadedshaft 86. The rotor 84 comprises a series of magnetic north and southpoles alternating around the shaft 86. The rotor 84 can comprise anysuitable magnetic material, such as ferrite.

Application of an alternating or rotating magnetic field from outsidethe patient can cause the rotor 84 to revolve about the axis of theshaft 86. The threaded engagement between the rotor 84 and shaft 86causes the rotor 84 to translate in the axial direction of the shaft 86,the direction of translation depending on the sense of rotation ofrotor. In this way the externally magnetic field can be used to move therotor 84 along the shaft 86 to urge the actuator plate 80 against the AVgraft 16 to apply constriction thereto, or to release pressure from theactuator plate 80 and reduce the constriction when high flow through theAV graft 16 is required.

FIGS. 7 and 8 show two further embodiments of the constriction device 20of the invention which both operate thermally. Each device has anactuator comprising a movable member 90 and a flexible membrane 92 forconstricting an AV graft 16, the device being housed in an enclosure 94.

In the embodiment of FIG. 7, the actuator member 90 is connected to asheet 96 made of a heat-deformable material. This is shown in its normalstate at body temperature whereby the AV graft 16 is constricted toreduce the quiescent flow therethrough. On raising the temperature ofthe sheet 96 it deforms into the shape indicated by the dashed line 98thereby pulling on the actuator 90, 92 to reduce the constriction on theAV graft 16. The material of the sheet 96 may be a shape-memorymaterial, such as a so-called smart metal, or it could be a bimetallicstrip or any other suitable material that deflects on changingtemperature, or a shape memory material that is magnetically activated.

In the device of FIG. 8, the actuator member 90 is connected to adeformable membrane 100 which defines one surface of a liquid filledcapsule 102 containing a liquid with a low boiling point, such as justabove body temperature, for example around 39° C. Under normalconditions the capsule 102 contains liquid and the actuator 90, 92squeezes the AV graph 16 to reduce blood flow. On increasing thetemperature of the substance in the capsule 102 above its boiling point,at least some of the liquid vaporises which results in an overallincrease in volume of the contents of the capsule 102. This expansiondeflects the membrane 100 and a force is transmitted via the member 90to lift the flexible membrane 92 to relieve the constriction of the AVgraph 16. The position of the deformable membrane 100 when in this stateis indicated by the dashed line 104.

The devices 20 shown in FIGS. 7 and 8 may be provided with an optionalheater 106, such as an electrical resistance. When it is desired toincrease the blood flow through the AV graft 16, electric current ispassed through the heater 106 to raise the temperature of the sheet 96or liquid filled capsule 102 to move the actuator as described above.The electrical current may be provided by a battery associated with thedevice and controlled by signals from an external controller asdescribed with reference to FIGS. 4(a) and 4(b), or the electricalcurrent may be provided by a radio frequency converter which convertsradio frequency radiation into electrical power, without the need for aninternal battery, as also described with reference to FIG. 4(a) and4(b). Alternatively, the increase in temperature necessary to change thestate of the thermal device may be provided by an external heat source.This eliminates the need for the heater 106. The external heat sourcemay take the form of, for example, an infrared lamp directed onto theskin in the vicinity of the device 20. The heater 106 could also be anantenna which heats up when an appropriate electromagnetic field isapplied.

When high flow through the AV graft 16 is no longer required, such aswhen hemodialysis has been completed, power to the heater 106 is cutoff, or the external heat source removed. The sheet 96 or fluid filledcapsule 102 cools back to normal body temperature and returns to theconfigurations shown in FIGS. 7 and 8 in which the actuator 90, 92 issqueezing the AV graft 16.

All of the above described constriction devices are intended to bewholly implantable within the patient. The enclosures 52, 82, 94comprise a titanium, ceramic or plastic box and the dimensions of thesides in transverse cross-section may be in the region of 10 to 30 mm,the unconstricted diameter of an AV graft being typically 5 to 8 mm. Theflexible membrane 46, 92, in contact with the AV graft 16 may be a verythin (i.e. 20 to 60 μm thick) titanium sheet or a thicker titaniummembrane preferably with appropriate corrugations 47 to facilitatedeflection, as shown in plan view in FIG. 4(c). The corrugations can beseen in cross-section in FIGS. 4(a), 4(b), 7 and 8. The regionsurrounding the AV graft 16, but within the respective enclosure, suchas the region 110 shown in FIGS. 7 and 8 may contain a deformable, butincompressible, material such as gel to control the constriction of theAV graft 16.

FIG. 9 shows schematically a constriction, such as in an AV graft 16.The normal diameter of the vessel is D, the constricted diameter is d,and the constriction is applied over a length L. It is preferred thatthe method and devices of the present invention apply the constrictionover an elongate portion of the AV graft 16, for example as shown inFIG. 4(a). Preferably the length L is at least twice the originaldiameter D, and L may even be five to ten or more times the diameter D.The reasons for this are as follows. For a given flow rate Q through theAV graft 16, the viscous losses are proportional to LQ, whereas theturbulent losses are proportional to [(D/d)²−1]²Q². These two lossescontribute to the overall dissipation caused by the constriction whichresults in the pressure drop and reduced flow rate. An acute localisedconstriction produces much turbulence which can cause thrombosis orunwanted stenosis downstream at the venous anastomosis, or in the AVgraft itself if it is made of living tissue. The same overall flowreduction can be achieved by increasing the length of the constrictionto increase viscous loss, but reducing turbulent loss.

One way to increase the length of the constriction is to providemultiple constriction devices in series along the AV graft 16. Anothermethod is to provide a single elongate actuator within the device ormultiple actuators disposed along the length of the device.

FIG. 10 illustrates a further technique for reducing turbulence causedby the constriction, namely by controlling the profile of theconstriction such that abrupt transitions in diameter are avoided. Theprofile of the constriction can be controlled by providing a pluralityof actuators 120, each of which squeezes the AV graft 16 by a controlledamount. The actuators 120 may all be provided within a singleconstriction device, or each actuator 120 may be provided in arespective constriction device disposed in series along the AV graft 16.Alternatively, a single actuator of a predetermined profile may be usedto cause a desired constriction profile.

A further technique for favouring viscous dissipation over turbulentdissipation is illustrated with reference to FIG. 11. A transversecross-section of the unconstricted AV graft is approximately circular asshown in the centre of FIG. 11. Applying an isotopic force around theperiphery to squeeze the vessel approximately equally in all directionswould tend to reduce the cross-section of the vessel to be a circle ofsmaller diameter. However, viscous losses are related to the area of thewall of the vessel and hence to the perimeter of the cross-section. Bysqueezing the AV graft 16 unequally in different directions, theperimeter of the lumen can be maintained substantially constant inlength while reducing its cross-sectional area. Various exemplaryresulting shapes are shown in FIG. 11. The arrows illustrate thedirections and points of application of the squeezing force. The devicesaccording to the invention can achieve constrictions of these shapes bya variety of ways, such as having ridged actuators, or a plurality ofactuators applying pressure in different directions or surrounding theAV graft 16 by a gel to control the shape of the deformation.

A further feature of the invention is to adhere the outer surface of theAV graft to the actuator using a glue. According to Bernoulli'sequation, p+½ρυ²; is constant, where p is pressure, ρ is viscosity and υis flow velocity. At a constriction, the flow velocity increases tomaintain throughput. At sufficiently high velocity, the pressure givenby Bemoulli's equation can become lower than the external pressure onthe vessel or even become negative. Thus, at a constriction it ispossible for collapse of the vessel to occur because the reducedpressure sucks the walls inwards. The flow of course then stops and thevessel recovers, but vessel collapse is problematic and results inerratic flow conditions. Gluing the wall of the AV graft to the actuatorprevents collapse by maintaining a minimum diameter of the AV graft,even when constricted. AV graft collapse may also be prevented if theconstriction is appropriately shaped, as shown in some of the examplesin FIG. 11, to resist further buckling under reduced pressure.

As previously mentioned, in one arrangement catheters for extracorporealflow to and from the AV graft 16 may be provided on opposite sides ofthe device 20. In this case it can be beneficial to increaseconstriction of the graft during e.g. hemodialysis in order to augmentflow through the extracorporeal machine. For the rest of the time, theconstriction is still partially applied to alleviate the problems, suchas caused by turbulence, whilst keeping the vein matured.

The method and device of the invention can also be used with AVfistulas, in which case the flow control device is placed on the arteryor vein, just proximal or distal to the fistula, respectively.

A further preferred aspect of the invention, which can be used with anyof the above-described embodiments, is to incorporate a flow-measuringdevice into the variable flow control device 20. FIG. 3 illustrated anexternal flow or turbulence measuring device 24, 26, however, accordingto the present further embodiment, the implanted device incorporatesflow-measuring apparatus. The flow measured by the device may becommunicated, for example, via an antenna 32, to an external device togive a reading of the flow passing through the AV graft. Alternatively,or in addition, the flow measurement may be used internally within theimplanted device to control the constriction applied, using a feed-backloop, to regulate the flow.

Examples of two technologies that can be used in embodiments of the flowcontrol device for measuring flow are described below.

(1) Ultrasonic Flow Measurements.

A piezo-element emits ultrasound, which is reflected by the flowingblood, the reflected signals being slightly changed in frequency throughthe Doppler effect, thereby carrying information on velocity which isdetected. Referring to FIG. 4(a) by way of illustrative example, theinformation on velocity is transmitted from the implanted device 20 viaantenna 32 to the external antenna 30 and is then received and displayedby the external control unit 22.

(2) Electromagnetic Flow Sensor

The flowmeter according to this embodiment works on the principle ofFaraday's Law of Induction, which states that if a conductor is movedwithin a magnetic field, a voltage is induced at right angles to thedirection of movement in that conductor and at right angles to themagnetic field. The voltage generated is proportional to the averagevelocity of the moving conductor. The voltage signal U is proportionalto the product vDB, where U=voltage across the channel, v=conductoraverage velocity, D=distance between the electrodes and B=magnetic fluxdensity.

An example of this embodiment is illustrated in FIG. 12. The blood inthe vessel 16 acts as the moving conductor. A magnetic field B can beapplied by an external magnet. Preferably the magnetic field coming fromthe external antenna 30 is used. This is advantageous because iteliminates the need to instal magnets or other means of imposing amagnetic field. Preferably the magnetic field is alternating, in whichcase a different frequency of B is used than the one used for thecontrol of the flow control device 20. Thus the external control unit 22emits one frequency, for example, for the telemetric control of a motor36 and for power generation, and another frequency for creating themagnetic field B required for the flow measurement.

The voltage measuring electrodes measuring 120 are placed perpendicularto B and v, and with a precisely known separation D. The EMF generatedbetween the electrodes 120 is sensed by a voltage measuring device 122.For improved measurement sensitivity, the voltage measuring device 122is tuned to the frequency of the externally applied magnetic field B.The electrodes 120 can be encapsulated either in the main box of thedevice 20 or in an auxiliary chamber next to the main box.

All of the preceding methods and devices according to the invention havebeen described in terms of application to an AV graft. However, asmentioned in the introduction, they can also be applied to the variableflow control of other bodily vessels, by which is meant a generallytubular structure that transports material in the body, such as a bloodvessel, lymph vessel, vessel in the digestive tract, vessel in theurinary tract, vessel in the reproductive tract, and so on. The bodilyvessel can be natural, or a graft, such as an autologous graft or asynthetic graft. Two further exemplary embodiments of applications otherthan to AV grafts will now be described.

(A) Hypoplastic Left (or Right) Heart Syndrome.

In this condition, blood is supplied by only a single ventricle of theheart. Referring to FIG. 13, pulmonary circulation must often be assuredby providing a shunt 200 connecting the subclavian or innominate artery202 to the right or left pulmonary artery 204. This is also known as themodified Blalock-Taussig shunt. The shunt 200 itself is a vasculargraft, such as a PTFE tube. Survival of the patent is often dependent onthe optimal distribution of flow between the shunt 200 and the aorta206.

According to the present invention, a variable flow control device 20 isplaced on the shunt 200. The shunt 200 drives flow from the systemiccirculation in the innominate artery 202 to the pulmonary circulation inthe pulmonary artery 204. The variable flow control device 20 is, forexample, according to any one of the above described devices. The flowcontrol device 20 enables the flow in the shunt to be regulated andaccording to a method of the invention, the flow in the shunt iscontrolled to equilibrate the repartition of flow between the systemicand pulmonary circulation.

(B) Esophageal Banding or Replacement of the Esophagus Valve.

The valve at the end of the esophagus connecting the esophageal tube tothe stomach may fail, causing re-entry of food from the stomach to theesophagus and consequent discomfort to the patent. Also, for thetreatment of obesity, sometimes a banding at the end of the esophagusmay be surgically placed. The banding causes a localised restriction tothe esophageal tube. Banding is not a precise procedure and is notadjustable without further abdominal surgery. According to the presentinvention, a variable flow control device, such as embodied above, islocated on the esophagus to alleviate either of these problems. Thedegree of esophageal restriction can be easily controlled telemetricallyto allow controlled passage of food into the stomach when required butto restrict it at other times or to prevent re-entry of food from thestomach into the esophagus.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described. The description is not intended to limit theinvention.

1. A device for controlling flow in an AV graft or AV fistula used forvascular access for an extracorporeal circuit, said device comprising:a) an actuator for applying partial constriction to a vessel, to providea reduced flow through said AV graft or AV fistula, when flow throughsaid extracorporeal circuit is to occur; and b) an adjuster for changingthe degree of construction, to modify the flow through the AV graft orAV fistula, when flow through said extracorporeal circuit is to occur.2. A device according to claim 1, wherein said vessel is said AV graft.3. A device, for controlling flow in a bodily vessel, said deviecomprising: an actuator for adjustably constricting said bodily vessel;and a rotatable member for driving said actuator.
 4. A device accordingto claim 3, further comprising a motor, said rotatable member being adrive shaft of said motor.
 5. A device according to claim 4, whereinsaid motor is an electrical micromotor.
 6. A device according to claim4, wherein said rotatable member comprises a rotor having a plurality ofmagnetic poles, such that rotation of said rotor can be induced by anexternally applied varying magnetic field.
 7. A device according toclaim 6, wherein said rotor is threadedly engaged with a shaft such thatrotation of said rotor causes translational motion between said rotorand shaft.
 8. A device, for controlling flow in a bodily vessel, saiddevice comprising: a deformable member which is reversibly deformable bya change in temperature or magnetic field; and an actuator acted on bysaid deformable member for adjustably constricting said bodily vessel,wherein said deformable member is deformable between a first state inwhich said actuator applies constriction to said bodily vessel, and asecond state in which said actuator reduces said constriction of saidbodily vessel.
 9. A device, according to claim 8, wherein saiddeformable member comprises a shape-memory material.
 10. A device,according to claim 8, wherein said deformable member comprises a capsulecontaining a substance, wherein said substance is substantially in aliquid phase when said member is in said first state, and said substanceis at least partially vaporizable to deform said member to said secondstate. 11-83. (canceled)